Constructing%20a%20Kinetics%20Database%20in%20Heterogeneous%20Catalysis

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Constructing a Kinetics Database in Heterogeneous
Catalysis
F. H. Ribeiro, J. M. Caruthers, W. N. Delgass,
K. T. Thomson, V. Venkatasubramanian Dept. of
Chemical Engineering, Purdue University
NSF Workshop, Washington, April 19-21, 2004
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Outline

• Analogy with homogeneous reactions
• Definitions and concepts in heterogeneous
  catalysis
• Challenges in setting a data base
• Suggestions

Catalysis is a kinetic phenomenon
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How to study kinetics?
H2 Br2 ? 2 HBr
Same general principle applied to
Langmuir-Hinshelwood kinetics
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Rate constants from NIST database 17
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Reaction rate simulation
Numerical solution is easy to implement, provides
perfect fitting
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Kinetic modeling

• Can calculate rates, coverages, selectivities at
  any condition, even if the rds changes
• There is no equation
• But it needs the rate constants!
• Help needed from Surface science
• Theoretical methods

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The key tool for analysis TURNOVER RATES
Turnover frequency ? Turnover rate

• TOR (Number of molecules reacted)
• (Number of sites) x (time)

Allows for comparison of data in different
catalysts in different laboratories
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Problems with defining a site

• What is a site?
• How do we count them?
• If they are not the same, what is the value of
  reporting a TOR (Langmuirian kinetics)?
• Need to be reported under reaction conditions.

Boudart, M. Turnover Rates in Heterogeneous
Catalysis. Chem. Rev. (1995), 95(3), 661-6.
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Reasons for using a TOR

• A method to compare catalyst performance,
  including the ones from different laboratories
• Study the influence of the arrangement of atoms
  on rates (structure sensitivity)
• It is a method to quantify heterogeneous
  catalysis (not perfect)

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Issues on developing a database in heterogeneous
catalysis

• Reaction mechanism changes with nature of surface

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Variation of rates on ammonia synthesis
NH3 activity/ arbitrary units
Ozaki, A. and K. Aika, Catalytic Activation of
Dinitrogen, in Catalysis Science and Technology,
J.R. Anderson and M. Boudart, Editors. 1981,
Springer Verlag New York. p. 87-158.
d-band occupancy/
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Issues on developing a database in heterogeneous
catalysis

• Reaction mechanism changes with nature of surface
  
• Not only with the nature of metal but also
  surface arrangement

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The rate is dependent on surface structure
Strongin, D.R., Carrazza, J., Bare, S.R.,
Somorjai, G.A., J. Catal. 103, 213 (1987).
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Practical catalysts are composed of nanometer
size particles
Profile view images of Rh/SiO2. Particles are
nearly cubo octahedral and exhibit (111) and
(100) facets (from A. K. Datye U. New Mexico )
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Issues on developing a database in heterogeneous
catalysis

• Reaction mechanism changes with surface nature
• Not only with the nature of metal but surface
  arrangement
• Rate constants are a function of coverage

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Rate constant value depends on coverage
Heat of adsorption of CO on Pt(111)
Yeo, Y.Y. , Vattuone, L. and King, D.A., J. Chem.
Phys. 106 (1), 392, 1997
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Issues on developing a database in heterogeneous
catalysis

• Reaction mechanism changes with surface nature
• Not only with the nature of metal but surface
  arrangement
• Rate constants are a function of coverage

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The situation is not hopeless

• Surfaces are non-uniform but in practice surface
  coverage varies in a limited range. What is
  needed?
• In many examples a simple power rate law
• In more involved cases, a full kinetic analysis
• In rare cases, Monte Carlo analysis

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What to do for a tabulation?

• Tabulate turnover rates and reaction orders
• Useful and easy to use
• Problems
• Limited approach. Would have to tabulate
  reactions at all conditions
• Data scatter

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Ethylene Hydrogenation
Ribeiro et al. , Catal. Rev. - Sci. Eng.,39,
49-76, 1997
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Some Problems with TOR Measurements on Metals
Turnover rate for the hydrogenolysis of ethane on
Pt at 473 K
The data are scattered!
"Reproducibility of Turnover Rates in
Heterogeneous Metal Catalysis Compilation of
Data and Guidelines for Data Analysis" F.H.
Ribeiro, A.E. Schach von Wittenau, C.H.
Bartholomew, and G.A. Somorjai, Catal. Reviews -
Sci. and Eng. , 39, 49, 1997
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What to do for a tabulation?

• 2. Tabulate rate constants
• Compact and thus can solve more problems
• Problems
• Availability of data
• Kinetic analysis suite not available

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What to do for a tabulation?

• 3. Compute the rate constants
• Tabulate the rates constants and also the output
  from the ab initio calculations
• How to store the data?

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Recommendations

• Make available to the community a robust software
  that will allow for kinetic modeling with rate
  constants
• Encourage the archival on a national database of
• Turnover rates
• Rate constants experimentally determined
• Rate constant calculations and wave functions

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Acknowledgments

• Department of Energy
• Office of Science